In steps for semiconductor production, a treating liquid containing an acid (e.g., sulfuric acid) and hydrogen peroxide is used in silicon wafer pretreatment, etc. As this treating liquid is repeatedly used, accumulation of impurities such as silica and decomposition of the hydrogen peroxide proceed to lower the acid concentration, resulting in a decrease in treatment function. It is hence necessary to renew the treating liquid intermittently. Because of this, a waste liquid having a high acid concentration and containing residual hydrogen peroxide is discharged in a large quantity. The amount of this waste liquid is affected by the regulations of the industrial waste management law.
Consequently, it becomes important to develop a technique by which the hydrogen peroxide remaining in the waste liquid is safely decomposed to a recycling level and which enables the concentrated acid (e.g., sulfuric acid) in the waste liquid to be recovered and be recycled or sold. In cases when the hydrogen peroxide is decomposed and the remaining acid is thereafter treated by neutralization, then there are cases where separation of the sediment yielded by the neutralization becomes difficult because the sediment rises up due to the remaining hydrogen peroxide.
Examples of techniques for decomposing the hydrogen peroxide using a catalyst include the following patent documents 1 to 4.
Patent document 1 discloses a hydrogen peroxide decomposition catalyst produced by forming a porous alumina coating film on the surface of a metal plate and fixing noble-metal colloidal particles having a particle diameter of 5 nm or less to the porous alumina coating film.
Patent document 2 discloses a catalyst which is a hydrogen peroxide decomposition catalyst for decomposing hydrogen peroxide present in a liquid phase into water and oxygen and which includes a support made of an inorganic oxide material having pores and one or more active metals fixed to the support and including at least one metal selected from the group consisting of Pt, Pd, Ir, Ru, Rh, and Os, the layer where the active metals have been fixed in the vicinity of the surface of the support having a thickness of 0.01-0.25 mm.
Patent document 3 discloses activated carbon for hydrogen peroxide decomposition which is a material for hydrogen peroxide decomposition obtained by kneading and dispersing an activated-carbon precursor and either at least one metal selected from among Ag, Pt, Pd, Cu, and Fe or a compound of the metal, infusibilizing and/or carbonizing the resultant mixture, and then activating the mixture, and in which the content of the metal component is 0.01% by mass or higher.
Patent document 4 discloses a supported platinum-group metal catalyst which includes an organic porous anion exchanger and, supported thereon, nanoparticles of a platinum-group meal having an average particle diameter of 1-100 nm. This catalyst is characterized in that the organic porous anion exchanger has an interconnected-pore structure having both macropores communicating with one another and common openings (meso-pores) formed in the walls of the macropores and having an average dry-state diameter of 1-1,000 μm, has a total pore volume of 1-50 mL/g, has anion exchange groups evenly distributed therein, and has an anion exchange capacity of 0.5-5.0 mg-eq/g-(dry porous object) and that the amount of the supported platinum-group metal is 0.004-20% by weight in terms of dry-state amount.